The invention relates to a method for operating a four stroke diesel engine in accordance with the preamble of the independent claim and to a four stroke diesel engine which is operated using this method.
During the operation of diesel internal combustion engines considerable amounts of polluting nitrogen oxides (NOx) arise in general. This also holds in particular for large diesel engines, such as are for example used as main drive aggregates for ships or as stationary plants for generating electricity. To protect the environment, therefore, considerable effort has been invested in developing diesel engines with significantly reduced NOx emission. The reduction of the NOx emission can for example be realized by means of selective catalytic reduction of the nitrogen oxides. In particular in large diesel engines however for reasons of space it is often very difficultxe2x80x94if at all possiblexe2x80x94to provide correspondingly dimensioned catalytic converter apparatuses. Therefore the development of so-called primary measures was concentrated on, in which the generation of the nitrogen oxides in the combustion chamber of the cylinder is reduced.
It is a known measure in diesel engines to inject water into the combustion chamber in addition to the fuel in order thereby to reduce the combustion temperature, in particular the peak temperature arising at the flame, through which a reduction of the NOx formation and thus of the NOx emission of the engine results. The water is injected into the combustion chamber of the cylinder during or already shortly prior to the combustion. In four stroke diesel engines it is usual to inject the water towards the end of the compression stroke e.g. at the same time as the fuel. The water can be introduced into the combustion chamber of the cylinder separately from the fuel; or a water-fuel emulsion is first produced, which is then injected into the cylinder.
Starting from this prior art, an object of the invention is to propose a simple method for operating a four stroke diesel engine, by means of which the nitrogen oxide emission of the four stroke diesel engine can be still further reduced in comparison with known operating methods without the necessity of a high apparative complexity or high costs.
The method for operating a four stroke diesel engine which satisfies this object is characterized by the features of the independent method claim.
Thus in accordance with the invention a method is proposed for operating a four stroke diesel engine, in which method water is injected into a cylinder in which a reciprocable piston is arranged, and which cylinder has at least one outlet for conducting off exhaust gases as well as at least one intake for supplying air, with the outlet being closed during the exhaust stroke of the piston so that the outlet is already closed before the piston reaches the upper idle dead center at the end of the exhaust stroke, and with water being injected into the cylinder when the piston is located in the region of the upper idle dead center.
Since the outlet of the cylinder is already closed before the piston reaches the upper idle dead center at the end of the exhaust stroke, in each case a portion of the exhaust gases which have arisen during the previous combustion process are retained in the cylinder. Water is injected into the still hot exhaust gases when the piston is located in the region of the upper idle dead center. From this there results a very good vaporizing of the injected water and an efficient cooling down of the exhaust gases which are still located in the cylinder. In the subsequent intake stroke fresh air flows into the cylinder and forms a mixture with the retained exhaust gases and the water vapor. This mixture has a reduced oxygen concentration in comparison with the fresh air. The mixture is compressed in the next compression stroke. At the end of the compression stroke fuel is injected into the mixture. As a result of the reduced oxygen concentration of the mixture and of the water vapor contained therein, significantly less nitrogen oxides arise during the subsequent combustion process, through which an operation of the diesel engine with low pollution is enabled. In addition the method in accordance with the invention can be carried out without significant additional cost and complexity, which is advantageous in regard to economical aspects.
Thus in the method in accordance with the invention a worsened or reduced scavenging, respectively, is intentionally realized in the sense that the scavenging of the exhaust gases out of the cylinder is incomplete. Through the combination of said retention of exhaust gases in the cylinder, which can be designated as internal exhaust gas recirculation, with the water injection the formation of nitrogen oxides during the combustion process is significantly reduced without concessions in regard to the economy of the motor being required. In the method in accordance with the invention both the retained exhaust gases and the water vapor contribute to reduce the NOx formation.
Since the water injection into the cylinder takes place when the piston is located in the region of its upper idle dead center, there further results the advantage that the cylinder liner is protected against a charging with water, so that the water can exert no disadvantageous or damaging influence on the oil film which is located on the surface of the cylinder.
The achieved cooling down of the hot exhaust gases which are retained in the cylinder can be controlled in a simple manner via the amount of the injected water.
In practice it has proved advantageous that the outlet is closed in such a manner that it is just completely closed at a crank angle of 30 xc2x0xc2x120xc2x0 ahead of the upper idle dead center. From this there results a favorable amount of exhaust gas which is retained in the cylinder. This amount of exhaust gas can be set or regulated respectively via the closing time points for the outlet.
The intake is preferably opened during the intake stroke, which means after passing through the upper idle dead center, so that the intake is still closed when the piston is located in the region of the upper idle dead center. Through this measure it is ensured that the intake or the intake passage respectively is not contaminated by the backwardly flowing exhaust gases. It has proved particularly advantageous to open the intake at a crank angle of 30 xc2x0xc2x120xc2x0 after the upper idle dead center.
The injection of the water preferably begins after the outlet is closed during the exhaust stroke and ends before the intake is opened during the intake stroke. Hereby, a particularly efficient cooling down of the hot exhaust gases which are still located in the cylinder can be achieved by the injected water and a particularly good vaporization of the injected water can be achieved.
From the practical point of view it is advantageous when the injection of the water takes place over a crank angle range of about 20xc2x0 ahead of the upper idle dead center to about 20xc2x0 after the upper idle dead center.
As a further advantageous measure for reducing the nitrogen oxides which arise during the combustion process water can additionally be injected into the cylinder during the compression stroke and/or during the combustion. Thereby, the combustion temperature in the combustion chamber can be reduced.
Said water can be injected into the cylinder during the compression stroke at the same time as the fuel.
In particular in regard to the apparative cost and complexity and the amount of space available at the cylinder it can be advantageous to inject the water into the cylinder by means of the same injection nozzle by means of which the fuel is injected. For this the injection nozzle can be executed as a so-called tandem nozzle which has two separate injection passages with in each case associated nozzle holes, with the one injection passage being used for the water injection and the other injection passage being used for the fuel injection. It is however also possible to use a conventional injection nozzle with only one injection passage. When the piston is located in the region of the upper idle dead center, water is injected into the cylinder through this injection nozzle, which brings about the additional advantage of a cooling of the injection nozzle. Then towards the end of the compression stroke, fuel or a fuel-water emulsion is injected into the combustion chamber of the cylinder through the same injection passage.
The water is preferably injected at a pressure of at least 200 bar in order to achieve as fine an atomization of the water as possible.
In accordance with the invention a four stroke diesel engine which is operated with a method in accordance with the invention is furthermore proposed.
Further advantageous measures and preferred embodiments of the subjects of the invention result from the subordinate claims.